Optical beam delivery configuration

ABSTRACT

An optical beam delivery configuration includes zoom lens means ( 30 ) and beam scanning means ( 44 ) defining an optical path ( 20 ) for a light beam. The beam scanning means is disposed after the zoom lens means in the direction of beam delivery. The zoom lens means ( 30 ) is arranged to receive a collimated incident light beam ( 23 ) on the optical path, and to be adjustable to determine the fluence of the beam when it is incident on the beam scanning means, while maintaining its collimation on exit from the zoom lens means. The beam scanning means ( 44 ) is arranged to laterally scan the beam at a downstream treatment location ( 50 ) while maintaining the beam&#39;s collimation and orientation at the location.

FIELD OF THE INVENTION

This invention related generally to an optical beam deliveryconfiguration, and has particular, though certainly not exclusive,application to the delivery of laser beams in medical laser systems.Such systems are used, for example, in a variety of ophthalmic surgicallaser treatments such as the correction of refractive errors byreshaping the corneal stroma in PRK (photorefractive keratectomy) andLASIK (laser in-situ keratomileusis), the sealing of leaky retinal bloodvessels, and the removal of debris from the posterior capsule of thelens after cataract surgery. The invention is also particularlyapplicable for any application that requires UV etching.

BACKGROUND ART

A fundamental requirement for beam delivery configurations in medicallaser systems is an accurately predictable beam profile at the treatmentsite, eg. an anterior or internal corneal treatment surface. Knowndelivery systems generally include, among other components, a beamshaping means, typically defining an aperture that sets the beamcross-section, a scanner, and a fluence control. The latter is set tocontrol fluence—energy density at a cross-section—to a fixed figure orat least below a predetermined limit for a given instrument and/orprocedure. Such limits are usually predetermined by regulatoryauthorities and adherence to them is generally a mandatory condition ofmarketing approval by such authorities.

International patent publication WO 98/57604 discloses a laser beamdelivery procedure and configuration in which the beam cross-section isvaried during scanning, typically by being progressively increased asthe surface being ablated is scanned in a predetermined pattern.Suitable scanning apparatus for this purpose is described ininternational patent publication WO 98/04303: that system has theparticular benefit of maintaining the beam collimated and parallel to afixed direction as it is laterally scanned. However, perfect collimationis often disturbed by at least two effects. Firstly, there is thedisturbance arising from fluence control downstream of the scanner.Secondly, many laser device beam outputs are significantly variable intheir cross-section and energy profile, and this causes still furthervariations at the fluence control as the latter in turn compensates forbeam fluctuations.

The overall result of these imperfections is that, while a laser breamdelivery system may incorporate a scanner arrangement in which the beamtheoretically remains optically collimated as it is scanned, the systemis still essentially height or z-axis sensitive, ie. the actual beamcross-section is dependent to some degree on the exact location of thetreatment surface on the optical path or axis. In consequence, apredictable outcome of the procedure is dependent on having thetreatment surface at an accurate location and on taking steps to preventz-axis movement of the surface during the procedure. This adds to thecomplexity and sensitivity of typical ophthalmic laser surgerytreatments: it would be preferable to provide an optical beam deliverysystem in which height or z-axis sensitivity was reduced, while makingallowance for the reality of beam fluctuations in laser device outputs.

SUMMARY OF THE INVENTION

The invention provides, in one aspect, an optical beam deliveryconfiguration including:

-   -   zoom lens means and beam scanning means defining an optical path        for a light beam, said beam scanning means being disposed after        said zoom lens means in the direction of beam delivery;    -   wherein said zoom lens means is arranged to receive a collimated        incident light beam on said optical path, and to be adjustable        to determine the fluence of the beam when it is incident on the        beam scanning means, while maintaining its collimation on exit        from the zoom lens means; and    -   wherein said beam scanning means is arranged to laterally scan        the beam    -   at a downstream treatment location while maintaining the beam's        collimation and orientation at the location.

Preferably, the configuration further includes beam shaping means insaid optical path for determining the cross-sectional shape,perpendicular to the optical path, of said light beam. The beam shapingmeans is preferably disposed between the zoom lens means and the beamscanning means, and may typically comprise a variable aperture such as avariable iris for varying the beam-diameter.

The zoom lens means is preferably a three lens system including a firstlens and, downstream thereof, a pair of lenses being a converging lensand a diverging lens respectively, which pair of lenses are arranged tomove along the optical path relative to the first lens, with a fixedspatial relationship between the lenses of the pair, for determining thefluence of the beam.

To reduce or minimise the effect of back reflections in the zoom lensmeans, said pair of lenses are preferably a piano-convex lens and aconcavo-plano lens, the former being upstream of the other relative tothe direction of the beam.

The beam scanning means may substantially be as described in theaforementioned international patent publication WO 98/57604.Alternatively, the beam scanning means may include one or moreconverging or convex lenses selected and positioned having regard totheir focal lengths so that one or more lenses may be translatedlaterally to effect scanning while maintaining the beam's collimationand orientation at the treatment location.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a laser beam deliveryconfiguration according to an embodiment of the invention;

FIG. 2 is a not-to-scale diagram of the principal components of part ofFIG. 1; and

FIG. 3 is a simple optical ray diagram depicting the imaging propertiesof the lenses D, E shown in FIGS. 1 and 2.

EMBODIMENTS OF THE INVENTION

FIG. 1 is an optical ray diagram for a laser beam delivery configurationaccording to an embodiment of the invention. The optical delivery is fora laser vision correction system and, although generally schematic andhighly simplified, depicts correctly the relative locations of thecomponents along the optical path.

The system is particularly suited for delivering a pulsed laser beamfrom a solid state laser such as an Nd:YAG laser 10 along an opticalpath 20 to a treatment location, in this case the cornea of an eye 50 ofa patient, who would typically be lying on an adjacent bed. Path 20includes segments in a post or other structure adjacent the bad and inan overhead cantilevered arm from which the beam is directed downwardlyat 21 to the eye. The changes of direction in the path are defined by aseries of mirrors 12, 13, 14, 15, 16 but it is emphasised that theactual folding and direction of the optical path may be quite differentfrom that illustrated according to the overall structural layout of theinstallation. The diagram shows only certain optical components ofrelevance to the present invention: the system will include many otherelements that are normally to be found in this kind of equipment, eg. asurgical microscope for viewing the procedure, a fixation device forholding the patients gaze, and optics for detecting and responding tomovement of the eye on which the operation is being performed.

A typical procedure carried out with the illustrated system is visioncorrection by selectively and controllably reshaping a corneal surfaceby photoablation of tissue. A particularly suitable wavelength for thispurpose is 213 nm. To obtain this wavelength, the 1064 nm primary oroutput beam 17 of laser 10 is passed through a sequence of non-linearoptical crystals 18 to derive several harmonics, including the fifthharmonic wavelength 213 nm, of the fundamental wavelength 1064 nm. Thisfifth harmonic is separated from the others by a dispersing prism 22 toform a collimated 213 nm laser beam 23 on optical path 20.

Beam 23 is passed in turn in its direction of delivery through a fluencecontrol in the form of a three-lens zoom telescope 30, beam shapingmeans in the form of an adjustable iris 40 and a scanner 44 formed by apair of confocal lenses D, E. Scanner unit 44 also has an iris imagingcapability with respect to iris 40, as will be explained subsequently.

Variable iris 40 determines the cross-sectional shape and diameter ofbeam 23 and of the beam actually delivered to the eye and, during aprocedure, may typically be varied as the beam is scanned. Normally,iris 40 and scanner 44 are cooperatively controlled by a predeterminedprogram to place pulses onto the corneal surface in order to produce aspecific refractive outcome.

The form and arrangement of the fluence control 30 and scanner 44 aredetailed in FIG. 2, which is a not-to-scale optical ray diagram of thesecomponents of the beam delivery system. Zoom telescope 30 has, insequence along the optical path 20, a plano-concave lens A, apiano-convex lens B and a concave-plano lens C. The telescope works byproviding lenses B, C with movement along the optical path to alter thebeam magnification, thus varying the fluence. Lenses B, C are moved bytranslation unit 32, and must move with a fixed spatial relationship tomaintain the beam collimation of the output 19 from lens C. This type ofmovement may be achieved with a CAM system, a direct drive or a lineardrive as translation unit 32, and also allows any value of magnificationto be obtained within the prescribed range. The illustrated three-lenstelescope is a development in principle of the classical two-lensGalilean telescope, in which the focal lengths of the negative andpositive lenses must coincide spatially and the magnification of thesystem is given by the ratio of the focal lengths of the two lenses. Inthe depicted arrangement, the lenses A and B can be viewed as forming asingle lens of variable power. That is, as lens B is moved, the focallength of, the equivalent lens AB is altered. Therefore, in order tomaintain the condition of coincident focal points, the third lens C mustalso be moved in unison with lens B.

It has been found preferable that lenses B and C be as shown rather thanthe converse, convex-piano and piano-convex concave respectively. Withthis latter arrangement, back reflections on lenses B, C are focussedback to lenses A, B respectively, risking an intense damaging spot onthe upstream lens.

Scanner 44 comprises a pair of convex lenses D, E in the form of aKeplerian telescope, ie. the two lenses have coinciding focal points atan intervening point 48. If the two lenses have equal focal lengths, thebeam diameter is unchanged at the output but it is preferred that amagnification factor 4:3 is produced so that the final lens E is theonly lens in the sequence A to E which is exposed to the full fluence.It has been found that the typical fluence employed, 170 mJ/cm² canreduce the life of the optical components so there is advantage inreducing the fluence exposure at lenses C, D to 9/16 of the finalfluence.

Beam scanning is achieved by off-axis translation, by translation unit49, of lens D, resulting in a lateral movement of the beam at thetreatment surface, indicated in FIG. 2 as a treatment plane TP. Whilethis type of scanning does not distort the beam through the introductionof aberrations, the position of TP becomes critical in terms of theamount of lateral movement. It may be preferable to scan both lenses Dand E in unison, which would provide lateral movement to parallel paths.

Scanner 44 also possesses the potential for iris imaging, as illustrateddiagrammatically in FIG. 3. An object placed in the front focal plane oflens D will be imaged in the back focal plane of lens E. Accordingly,the position of TP can be chosen to be in the imaging plane and the irisplaced in the object plane. This has the disadvantage, however, ofgoverning the position of TP.

By the particular sequence of fluence control and scanner, and with theiris preferably interposed between, the beam produced at the eye isreliably collimated and of predictable cross-section and fluencenotwithstanding the variations in the laser beam, thus substantiallyeliminating height sensitivity. Moreover, this configuration is achievedwith relatively few optical components in a simple layout.

1. An optical beam delivery configuration including: zoom lens means andbeam scanning means defining an optical path for a light beam, said beamscanning means being disposed after said zoom lens means in thedirection of beam delivery; wherein said zoom lens mean is arranged toreceive a collimated incident light beam on said optical path, and to beadjustable to determine the fluence of the beam when it is incident onthe beam scanning means, while maintaining its collimation on exit fromthe zoom lens means; and wherein said beam scanning means is arranged tolaterally scan the beam at a downstream treatment location whilemaintaining the beam's collimation and orientation at the location. 2.An optical beam delivery configuration according to claim 1 furtherincluding a beam shaping means in said optical path for determining thecross-sectional shape of said light beam perpendicular to said opticalpath.
 3. An optical beam delivery configuration according to claim 2wherein said beam shaping means is disposed in said optical path betweensaid zoom lens means and said beam scanning means.
 4. An optical beamdelivery configuration according to claim 2 wherein said beam shapingmeans comprises a variable aperture for varying the beam diameter.
 5. Anoptical beam delivery configuration according to claim 1 wherein saidzoon lens means is a three lens system including a first lens and,downstream thereof in said direction of beam delivery, a pair of lensesbeing a converging lens and a diverging lens respectively, which pair oflenses is arranged to move along the optical path relative to the firstlens, with a fixed spatial relationship between the lenses of the pair,for determining fluence of the beam.
 6. An optical beam deliveryconfiguration according to claim 5 wherein said lenses of said pair area plano-convex lens and a concavo-plano lens, the plano-convex lensbeing upstream of the concavo-plano lens relative to said direction ofbeam delivery.
 7. An optical beam delivery configuration according toclaim 1 wherein said beam scanning means includes one or more convergingor convex lenses selected and positioned having regard to their focallengths so that one or more lenses may be translated laterally to effectscanning while maintaining the beam's collimation and orientation at thetreatment location.
 8. An optical beam delivery configuration accordingto claim 1, for receiving a primary laser beam generated by a solidstate laser and further including a sequence of non-linear opticalcrystals for deriving a harmonic of said primary laser beam as saidlight beam delivered via said zoom lens means and said beam scanningmeans.
 9. An optical beam delivery configuration according to claim 8further including a solid state laser for generating said primary laserbeam.
 10. An optical beam delivery configuration according to claim 1,in ophthalmic surgical laser apparatus.
 11. An optical beam deliveryconfiguration according to claim 10 wherein said ophthalmic surgicallaser apparatus is adapted for correction of refractive errors of theeye by reshaping of a corneal surface by photoablation of tissue.